In an internal combustion engine, a fuel injector is provided for delivering a charge of fuel to a combustion chamber prior to ignition. One known type of injector behaves as an inductive load and comprises an electromagnetic valve which is configured to open when an electrical current is supplied to the injector and to remain closed when the current is interrupted. It is important to control the timing and delivery volume of a fuel injection event, and in order to achieve this an injector is provided with an associated control unit and driver circuit.
One known injector drive circuit is shown in FIG. 1 and indicated generally at 100. A similar arrangement is described in US-A-20090192695. In FIG. 1, an injector 101 is controlled by a high side driver circuit 102 and a low side driver circuit 103. The high side driver circuit 101 comprises a high side driver 104 and a first (high side) transistor 105. An output of the high side driver 104 is connected to the gate of the first transistor 105. The low side driver circuit 103 comprises a low side driver 106 and a second (low side) transistor 107. An output of the low side driver 106 is connected to the gate of the second transistor 107. The first and second transistors 105, 107 act as switches and may comprise N-channel MOSFET (metal oxide silicon field effect transistor) devices.
The first (high side) transistor 105 is arranged electrically upstream of the injector 101 on a power source side. The second (low side) transistor 107 is arranged electrically downstream of the injector 101 on a ground side.
In the example of FIG. 1, the high side driver 104 controls the gate of the first (high side) transistor 105 in a PWM (pulse width modulated) mode according to a PWM control signal which is applied to an input of the high side driver 104. The drain of the first (high side) transistor 105 is supplied by a “boost voltage” and the amount of current flowing through the injector 101 is then controlled by the PWM signal corresponding to the on-off periods of the first (high side) transistor switch 105. An additional low side driver control signal applied to the input of the low side driver 106 controls the gate of the second (low side) transistor 107. Typically, a “freewheeling” diode 108 is connected between the injector 101 and ground for discharging the inductor current inside the injector 101.
In the case where the first (high side) transistor 105 is an N-channel MOSFET, its gate needs to be at least 7 V (typical threshold voltage) higher than the boost voltage in order to guarantee switch-on in saturation mode. To fulfill this requirement, the known technique of providing a bootstrap charge storage element or “bootstrap capacitor” 109 can be employed for control of the gate of the first (high side) transistor 105. The bootstrap capacitor 109 is connected across the positive and negative supply rails of the high side driver 104 and is charged up by a voltage source 110 through a diode 111. In the automotive application, the voltage source may be provided by the vehicle's battery. By maintaining a suitable voltage across the bootstrap capacitor 109 it is possible to generate a gate voltage sufficiently high to force the high side transistor 105 into its saturation mode.
The use of the diode 111 prevents charge flowing back from the bootstrap capacitor 109 to the voltage source 110. However, the use of the diode is unsatisfactory in the case where the battery voltage is low, for example, during cranking of the engine. This is because of the inherent threshold voltage of the diode which causes a voltage drop across it, which in the case of low battery voltage, may mean that an insufficient voltage is applied to the bootstrap capacitor 109 for the purpose of driving the high side transistor 105 into saturation. There is a further automotive requirement for the injector control and drive circuitry to have high immunity to electrical interference. In particular, the bootstrap capacitor should not be discharged on occurrence of an electrical interference event. This can be a particular problem during idle mode where both high side and low side transistors 105, 107 are in the OFF mode and the high impedance state of the driver circuitry renders it more susceptible to interference.